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6. A note on callsigns, addresses and things before we start.

Each AX.25 and NetRom port on your system must have a callsign/ssid allocated to it. These are configured in the configuration files that will be described in detail later on.

Some AX.25 implementations such as NOS and BPQ will allow you to configure the same callsign/ssid on each AX.25 and NetRom port. For somewhat complicated technical reasons Linux does not allow this. This isn't as big a problem in practise as it might seem.

This means that there are things you should be aware of and take into consideration when doing your configurations.

  1. Each AX.25 and NetRom port must be configured with a unique callsign/ssid.
  2. TCP/IP will use the callsign/ssid of the AX.25 port it is being transmitted or received by, ie the one you configured for the AX.25 interface in point 1.
  3. NetRom will use the callsign/ssid specified for it in its configuration file, but this callsign is only used when your NetRom is speaking to another NetRom, this is not the callsign/ssid that AX.25 users who wish to use your NetRom `node' will use. More on this later.
  4. Rose will, by default, use the callsign/ssid of the AX.25 port, unless the Rose callsign has been specifically set using the `rsparms' command. If you set a callsign/ssid using the `rsparms' command then Rose will use this callsign/ssid on all ports.
  5. Other programs, such as the `ax25d' program can listen using any callsign/ssid that they wish and these may be duplicated across different ports.
  6. If you are careful with routing you can configure the same IP address on all ports if you wish.

6.1 What are all those T1, T2, N2 and things ?

Just as not every ham is an engineer, not every AX.25 implementation is a TNC2. Linux uses nomenclature that differs in some respects from that you will be used to if your sole experience with packet is a TNC. The following table should help you interpret what each of the configurable items are, so that when you come across them later in this text you'll understand what they mean.

-------------------------------------------------------------------
Linux  | TAPR TNC | Description
-------------------------------------------------------------------
T1     | FRACK    | How long to wait before retransmitting an
       |          | unacknowledged frame.
-------------------------------------------------------------------
T2     | RESPTIME | The minimum amount of time to wait for another
       |          | frame to be received before transmitting
       |          | an acknowledgement.
-------------------------------------------------------------------
T3     | CHECK    | The period of time we wait between sending
       |          | a check that the link is still active.
-------------------------------------------------------------------
N2     | RETRY    | How many times to retransmit a frame before
       |          | assuming the connection has failed.
-------------------------------------------------------------------
Idle   |          | The period of time a connection can be idle
       |          | before we close it down.
-------------------------------------------------------------------
Window | MAXFRAME | The maximum number of unacknowledged
       |          | transmitted frames.
-------------------------------------------------------------------

6.2 Run time configurable parameters

The 2.1.* and 2.0.29+module kernels have a new feature that allows you to change many previously unchangable parameters at run time. If you take a careful look at the /proc/sys/net/ directory structure you will see many files with useful names that describe various parameters for the network configuration. The files in the /proc/sys/net/ax25/ directory each represents one configured AX.25 port. The name of the file relates to the name of the port. The structure of the files is as follows:

No.     Name                    Meaning                         Default
1       IP Default Mode         0=DG 1=VC                       0
2       AX.25 Default Mode      0=Normal 1=Extended             0
3       Allow Vanilla Connects  0=No 1=Yes                      1
4       Backoff                 0=Linear 1=Exponential          1
5       Connected Mode          0=No 1=Yes                      1
6       Standard Window         1  <= N <= 7                    2
7       Extended Window         1  <= N <= 63                   32
8       T1 Timeout              1s <= N <= 30s                  10s
9       T2 Timeout              1s <= N <= 20s                  3s
10      T3 Timeout              0s <= N <= 3600s                300s
11      Idle Timeout            0m <= N                         20m
12      N2                      1  <= N <= 31                   10
13      AX.25 Frame Length      1  <= N <= 512                  256
14      Max Queue               1  <= N <= 20                   2
15      Digipeater Mode         0=None 1=Inband 2=XBand 3=Both  3

In the table T1, T2 and T3 are given in seconds, and the Idle Timeout is given in minutes. But please note that the values used in the sysctl interface are given in internal units where the time in seconds is multiplied by 10, this allows resolution down to 1/10 of a second. With timers that are allowed to be zero, eg T3 and Idle, a zero value indicates that the timer is disabled.


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